Clinical magnetic resonance (MR) imaging techniques typically detect a signal from hydrogen protons.
The detected hydrogen protons are predominantly part of water, or part of organic molecules such as proteins, carbohydrates, and fat, or part of introduced inorganic-organic complexes, such as silicone.
The respective signal intensities of the various hydrogen proton pools in an imaging voxel results from a combination of their spin density, longitudinal and transverse, relaxation time (T1 and T2, respectively), and the parameters of the imaging sequence used.
By exploiting the particular characteristics of hydrogen atoms (hydrogen having a single proton (1H) nucleus), MR imaging (MRI) provides good contrast between soft tissues, according to the chemical form and local microscopic environment of the hydrogen containing species in the tissues, such as silicone or lipid (fat) molecules.
The electronic shielding of the hydrogen protons in macromolecules such as fat or silicone is greater than that experienced by hydrogen protons in water. This results in different microscopic magnetic field environments for the hydrogen protons, and subsequently different hydrogen proton resonant frequencies for the different hydrogen-containing chemical species—referred to as chemical shift.
For example, fat is known to have a complex spectrum with multiple peaks owing to its various hydrogen proton chains, for which the largest peak is shifted downfield by ≈3.5 ppm from the peak for water.
A known MR imaging modality relies on suppression of the fat peak compared to the water peak. A more advanced approach for fat suppression relative to the water peak is to excite the water peak directly via spatial spectral pulses, rather than suppress the fat peak.
Chemical shift-based water-fat separation methods that successfully and accurately separate the signals from water and fat provides a mechanism by which the fatty infiltration of organs can be quantified in a variety of disease conditions. For example, non-alcoholic fatty liver disease (NAFLD) is a major cause of chronic liver disease in the USA, affecting approximately one third of the US population. The current benchmark for the diagnosis of NAFLD is liver biopsy, which is expensive, risky, and suffers from high sampling variability, greatly limiting its clinical utility. Therefore, there is a great need for noninvasive biomarkers such as imaging, not only for early detection of disease, but also to reliably quantify the severity of disease.
In the case where two distinct MR chemical species can dominate the MR signal within a given volume, it can be highly problematic to ascribe a given signal component to a given species, unless the relative proportions or average rate at which the signal decays or recovers for a given species is known relative to the other species a priori, or other anatomical reference information is available to make the distinction. This is classically seen in spin-echo imaging where multi-exponential decay curve fitting, is attempted on signal intensity data collected at different echo times to derive the characteristic transverse relaxation rates and relative proportions of two or more MR species contributing to the MR signal. A similar situation arises in gradient-echo imaging where a sinusoidal oscillation is superimposed on the MR signal decay curve owing to differences in the characteristic MR frequencies of the chemical species contributing to the MR signal. An example of this occurs in the gradient-echo imaging of a volume containing both water and fat, where it is desired to isolate the water and fat signals and/or determine the relative percentage contributions of each. When a water/fat signal model assumes a characteristic MR frequency for each species and is being fit to magnitude image data, then assignment of a given signal component to either water or fat cannot be reliably performed, unless it is known that a given tissue cannot exceed 50% fat, or other anatomical information can be used to make the distinction.
The water/fat ambiguity problem that arises in the fitting of a bi-exponential signal decay model to gradient-echo image data arises from over-simplification of the model where fat is characterised by a single resonant frequency. The ambiguity has the potential for being resolved by using a multi-peak spectral model for fat instead that accounts for contributions to the MR signal from the major hydrogen proton groups on the fat molecule, but utility beyond the 50% fat fraction has not been demonstrated by magnitude-based techniques employing this approach (Yokoo et al in 2011).
The use of complex data with both magnitude and phase can help overcome the water/fat ambiguity problem, but also requires the determination of a field map, with complex data being further sensitive to phase errors from various sources (Yu et al in 2008, Reeder et al in 2009, US patent application published as US 2011/0254547 to Reeder et al).
However, ambiguity can still arise if the initial estimates for the relative proportions of fat and water and their relaxation rates more closely reflect the actual values for the other species, as the fit is then likely to converge to an incorrect assignment of each component. This result can easily arise in bi-exponential signal decay modelling, where there are many local minima for the mean square error of the difference between the fit and the data, to which the fitting routine may be drawn as representing the global minimum solution, and thus falsely ascribe the fat component as the water component, and vice-versa.
Regarding water-silicone separation MR imaging techniques, it will be appreciated that using one or more forms of the present invention enables the presence of a silicone rich species to be assessed, thereby identifying whether leakage or damage of an implant has occurred. This can provide a vitally important non-invasive safety screening technique for breast and other silicone based implants.
It is desirable of the present invention to provide a magnetic resonance technique that enables chemical separation by quantifying the proportion of one selected species, such as fat or silicone, from water in tissue.